a. Field of the Invention
The invention relates to an illumination system and a projection apparatus having the illumination system.
b. Description of the Related Art
Currently, in a projection apparatus an ultra high pressure lamp capable of emitting white light may cooperate with a color wheel to produce red light, green light, and blue light in succession and to display color images as a result. Further, in the recent trend, red light emitting diodes, green light emitting diodes, and blue light emitting diodes are used as a light source for a projection apparatus.
Referring to FIG. 6, in a projection apparatus 100 where a red light emitting diode, a blue light emitting diode, and a green light emitting diode are used as a light source, an X-mirror 102 is commonly used to combine different color beams. The X-mirror 102 includes, for instance, a red dichroic mirror 102a and a blue dichroic mirror 102b cross to each other. A red light beam emitted from a red LED 104R is reflected by the red dichroic mirror 102a, a blue light beam emitted from a blue LED 104B is reflected by the blue dichroic mirror 102b, and a green light beam emitted from a green LED 104G passes through the red dichroic mirror 102a and the blue dichroic mirror 102b. Hence, the X-mirror is allowed to guide the red light beam, the green light beam, and the blue light beam coming from different directions to propagate in an identical direction, and then these light beams are homogenized by a fly-eye lens 106. Thereafter, the red light beam, the green light beam, and the blue light beam are reflected by a reflective mirror 108 and modulated by a digital micro-mirror device 110 to form a colored image beam. Finally, the colored image beam enters a projection lens 112. However, in the X-mirror 102, an adhesive region (i.e. overlapped region) between the red dichroic mirror 102a and the blue dichroic mirror 102b fails to deflect the red light beam, the green light beam, and the blue light beam to result in the loss of light. Besides, an area of the adhesive region compared with a cross-sectional area of a light beam emitted from a light-emitting diode becomes larger when the light-emitting diode instead of an ultra high pressure lamp is used. This may result in higher loss of light. Further, since the red light beam, the green light beam, and the blue light beam are incident to the X-mirror 102 from three different directions, the space-efficiency of components in a projection apparatus is poor and results in a bulky projection apparatus.
Further, as shown in FIG. 7, U.S. Pat. No. 7,201,498 discloses a light-mixing system where light beams emitted from LEDs 124B, 124G, and 124R are reflected by three dichroic mirrors that are not parallel to each other and then are guided into an objective 126. However, such design merely provides the function of combining different color beams but fails to disclose how to improve light-utilization efficiency and reduce occupied space of a projection apparatus. Similarly, the design of three dichroic mirrors not parallel to each other is also disclosed in U.S. Pat. No. 6,910,777 and U.S. Pat. No. 6,987,546. However, these designs similarly do not disclose how to improve light-utilization efficiency and space-utilization efficiency.